Read the Passage carefully and answer the questions.
The carbonyl group of aldehydes and ketones is highly polar. The positively charged carbon is readily attacked by electron-rich nucleophile and the negatively charged oxygen atom is attacked by an electro-deficient electrophile. Aldehydes and ketones undergo nucleophilic addition reactions in the presence of acid or a base. The mechanism involves attack of a nucleophile on the electron deficient carbonyl carbon to form a tetrahedral intermediate. A new bond is formed between the carbon and oxygen. Protonation of the anion furnished by the reaction medium yields a neutral addition product. The addition product is same whether the reaction is catalyzed by acid or a base. For example in the presence of base aldehydes and ketones react with hydrogen cyanide to form cyanohydrin.
Which of the following reactions is not a nucleophilic addition reaction of aldehydes and ketones?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Addition of sodium borohydride
In a nucleophilic addition a reagent H–Nu (or its ions) adds across the C=O bond: the nucleophilic part attaches to the carbonyl carbon and the electrophilic part (usually \( \ce{H^{+}} \)) to oxygen, and the reagent itself becomes part of the product. Typical examples are the additions of HCN, sodium hydrogensulphite, Grignard reagents, alcohols and water.
Sodium borohydride, \( \ce{NaBH4} \), is a reducing agent. It does not add as a reagent across the C=O bond; it only reduces the carbonyl compound, aldehydes to primary alcohols and ketones to secondary alcohols. It is therefore studied as a reduction, not as one of the nucleophilic additions:
\( \displaystyle \ce{RCHO} \xrightarrow{\ce{NaBH4}} \ce{RCH2OH} \qquad \ce{R2CO} \xrightarrow{\ce{NaBH4}} \ce{R2CHOH} \)
Option (1) Addition of water — A nucleophilic addition reaction. Water adds across the C=O bond to give a gem-diol (hydrate), \( \ce{R2C(OH)2} \); for example, methanal in water exists mainly as \( \ce{CH2(OH)2} \).
Option (2) Addition of sodium Hydrogen Sulphite — A nucleophilic addition reaction. The hydrogensulphite ion adds to give a crystalline bisulphite addition compound, \( \ce{R2C(OH)SO3^-Na^+} \), used to separate and purify aldehydes.
Option (3) Addition of alcohol — A nucleophilic addition reaction. One molecule of an alcohol adds to an aldehyde to give a hemiacetal, which reacts with a second molecule (dry HCl) to give an acetal.
Option (4) Addition of sodium borohydride — Not a nucleophilic addition (a reduction). Sodium borohydride only reduces the carbonyl group; it does not add across C=O as a reagent.
NCERT (2026-27 edition):
Reduction – “… Reduction to alcohols: Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4) as well as by catalytic hydrogenation (Unit 7, Class XII).”